Akama Makoto

A railway wheel flat occurs when a wheel locks and slides along the rail, forming a white etching layer (WEL) beneath the flat surface. Rolling contact fatigue cracks often initiate within the WEL and propagate into the wheel, eventually resulting in spalling during service. In this study, twin-disc wear-fatigue tests were conducted to investigate the effect of reversing the wheel rolling direction on crack propagation behaviour leading to spalling and wear. Finite element analyses (FEA) were also performed to interpret the experimental observations. Under lubricated conditions, cumulative wear increased because of WEL spalling after the rolling direction of the wheel specimen was reversed. The FEA indicated that reversing the rolling direction caused lubricant to become trapped within cracks propagating along the WEL edge, reducing the friction coefficient between the crack faces and promoting Mode II (shear-mode) crack growth, thereby accelerating fatigue wear.

Cracks on the head of railway rails may initiate and propagate as a result of rolling contact fatigue (RCF) caused by repeated wheel–rail contact. These cracks can eventually lead to serious rail failures and accidents. To accurately predict the fatigue life of rails, it is essential to clarify the mechanisms of crack initiation, short-crack propagation, long-crack propagation, crack branching, and branch crack propagation. Accurate prediction of each stage would enable both enhanced railway safety and cost-effective maintenance planning. This study aims to develop a whole-life simulation model capable of predicting the complete life of RCF cracks in railway rails.

In this study, we conducted a series of twin-disc wear and fatigue tests and FE simulations of the tests. The results are as follows. In the wet conditions, the wear rate increased when the rolling direction of the wheel disc was reversed. In contrast, in the dry conditions, the wear rate was much higher than in the wet conditions, however, it changed little before and after the reversal. The Archard model could not explain the increase in wear rate after the reversal in wet conditions, because the model does not take into account the delamination wear induced by the fluid. Some discs after the test were sectioned and photographed to observe the crack morphologies and microstructural changes by optical metallography. In wet conditions, when the wheel disc was reversed, the cracks clearly tended to propagate parallel to the surface, and the resulting long and thin wear sheets delaminated. In this case, the crack is filled with fluid just before its mouth reaches the contact area between the wheel and rail discs, whereupon the mouth is sealed. Therefore, the fluid is trapped in the crack and acts as a lubricant to reduce the friction coefficient, and cracks tend to propagate easily and large delamination sheets are apt to form. In order to explain the increase in wear rate after the reversal under wet conditions, CTSDs were obtained by FE analyses. In the wet conditions, the ΔCTSD after the reversal was almost twice as large as that before the reversal. This means that the amount of crack propagation after the reversal is higher than that before the reversal. In contrast in the dry conditions, the ΔCTSD is almost the same before and after the reversal. Therefore, the amount of crack propagation before and after the reversal is considered to be almost the same.